1) Field of the Invention
The present invention relates to a process for producing trimellitic acid by liquid phase oxidation of pseudocumene as a starting material, and specifically to a process for producing trimellitic acid which comprises oxidizing pseudocumene as a starting material in water solvent and producing intermediate products step by step and oxidizing thereof, thereby producing trimellitic acid of a final product in a high yield.
2) Prior Art
Terephthalic acid has been industrially produced in many states by air oxidation of p-xylene in acetic acid solvent in the presence of a bromine-transition metal catalyst. In a liquid phase oxidation reaction employing aromatic hydrocarbons as raw material, it is indispensable to use acetic acid as a solvent in order to obtain aromatic polycarboxylic acids. When acetic acid is used as such solvent, loss of acetic acid occurs by combustion.
Pseudocumene is oxidized with air in the presence of a heavy metal catalyst in the same manner as other alkyl aromatic compounds, whereby trimellitic acid is produced. However, it is known that trimellitic acid thus produced forms a complex with a heavy metal(s) due to ortho structure of two carboxylic groups in trimellitic acid to cause deactivation of the catalyst, so that its yield is lower than that in other alkyl aromatic compounds.
As a conventional process for producing trimellitic acid, U.S. Pat. No. 3,491,144 discloses a process for producing trimellitic acid which comprises initially oxidizing pseudocumene in acetic acid in the presence of Co bromide and adding Co, Mn and bromide thereto and further oxidizing at 200xc2x0 C. or above.
Further, Japanese Patent Publication No.58-2222 discloses a process for producing trimellitic acid by oxidizing a polyalkyl-substituted aromatic aldehyde or oxide derivative thereof in water solvent with molecular oxygen.
In the production of trimellitic acid by oxidation of pseudocumene in acetic acid, loss of acetic acid as the solvent occurs by combustion. Further, there are some problems that when oxidation is performed in acetic acid solvent, it is avoidable to apply a batch wise. Thus, a process for producing trimellitic acid in a high yield without using acetic acid has been required.
On the other hand, the case where a polyalkyl-substituted aromatic aldehyde or oxide derivative thereof as a starting material is oxidized in water solvent has the defect that the polyalkyl-substituted aromatic aldehyde or oxide derivative thereof is expensive.
An object of the present invention is to provide a process for producing industrially and continuously trimellitic acid in a high yield by liquid phase oxidation of cheap pseudocumene as a starting material without using acetic acid as a solvent.
As a result to extensive studies to solve the prior art problems, the inventors have found that trimellitic acid can be obtained continuously in a high yield by oxidizing pseudocumene as a starting material and producing intermediate products step by step and oxidizing thereof and have accomplished the present invention.
That is, the present invention provides a process for producing trimellitic acid which comprises:
step A for oxidizing pseudocumene, thereby obtaining a reaction mixture comprising dimethyl benzoic acid, dimethyl benzyl alcohol, and dimethyl benzaldehyde
step B for separating dimethyl benzoic acid, dimethyl benzyl alcohol and dimethyl benzaldehyde from the reaction mixture obtained in step A,
step C for oxidizing dimethyl benzyl alcohol separated in step B, thereby obtaining dimethyl benzoic acid and dimethyl benzyl aldehyde and then feeding dimethyl benzoic acid and dimethyl benzyl aldehyde thus obtained to step B, and
step D for oxidizing dimethyl benzoic acid and/or dimethyl benzaldehyde separated in step B, thereby obtaining trimellitic acid.